Sensors are known which are able to detect a mechanical disturbance along one or more optical fibres in a cable or conduit, for example in order to detect a possible eavesdropping attempt or to otherwise provide for secure communication. However, the entire cable may be innocently disturbed, for example in the case of an underground cable, by a sound wave propagating through the ground.
It is known for sensors to operate using a form of interferometric principle based on speckle patterns. For this, a section of multimode fibre is splice-in at the locations to be sensed. As the light passes through the multimode section it excites a number of propagation modes which then interfere at the point where they are filtered by the transition back into the single-mode fibre. This mode-filtering of the speckle pattern is very sensitive to micro disturbance of the multimode fibre and results in a varying transmission loss through this section which is then detected. The problem with this approach is that it requires special fibres to be spliced in at all points to be sensitised and renders the fibre link unsuitable for standard transmission system use. It would be possible to make the entire link out of multimode fibre and filter the speckle pattern at the end, but this suffers from the same noise build up problem as the single mode case.